Epoxy resins' superior toughness, chemical resistance, heat resistance, adhesion and electrical properties have contributed to their wide use in electrical and structural applications and in protective coatings. An epoxy group (1,2-epoxide or oxirane), a three-membered cyclic ether group, characterizes the epoxy resins. A curing agent reacts with these monomers or prepolymers to produce high performance thermosetting plastics.
The diglycidyl ethers of bisphenol A are a common form of epoxy resin. They are produced by well known processes such as the reaction of dihydric phenols and epihalohydrin. In one such process the epihalohydrin and dihydric phenol react in the presence of a catalyst to produce a halohydrin-containing resin intermediate which is then reacted with a basic acting material, e.g. sodium hydroxide. Treatment of the resulting reaction mixture, such as by water-washing, removes residual catalyst and salt, producing a liquid epoxy resin.
Various dihydric phenols are employed; e.g. hydroquinone, resorcinol, catechol, and bisphenols. Suitable epihalohydrins which can be employed herein include, for example, epichlorohydrin, epibromohydrin, epiiodohydrin, methylepichlorohydrin, methylepibromohydrin, methylepiiodohydrin, mixtures thereof and the like. Suitable catalysts include, for example, quaternary ammonium compounds, quaternary phosphonium compounds, sulfonium compounds and the like.
Suitable quaternary ammonium catalysts include, for example, tetramethyl ammonium chloride, benzyl trimethyl ammonium chloride, triethanol ammonium chloride, tetraethanol ammonium hydroxide, dodecyl dimethylbenzyl ammonium naphthenate and the like. Suitable quaternary phosphonium catalysts include, for example, those quaternary phosphonium compounds disclosed in U.S. Pat. Nos. 3,948,855, 3,477,990 and 3,341,580 and Canadian 858,648 all of which are incorporated herein by reference. Other catalysts are ethyl triphenyl phosphonium iodide, ethyl triphenyl phosphonium bicarbonate, ethyl triphenyl phosphonium acetate-acetic acid complex, benzyl triphenyl phosphonium chloride, tetrabutyl phosphonium chloride, benzyl trimethyl ammonium chloride mixtures thereof and the like. Suitable sulfonium catalysts include thiourea catalysts such as, for example, tetramethyl thiourea; N,N'-dimethyl thiourea; N,N'-diphenyl thiourea; mixtures thereof and the like as well as thiodiethanol and other sulfonium precursors.
Also suitable as catalysts are the basic ion exchange resins such as, for example, DOWEX.RTM. MSA-1, DOWEX.RTM. 11, DOWEX.RTM. SBR, mixtures thereof and the like.
Specific processes for producing epoxy resins are described in these U.S. Pat. Nos. 4,313,886; 2,986,551; 3,069,434; 2,840,541; 3,221,032; 4,017,523; 4,751,280; and 4,008,133; and in various foreign references, including Great Britain 2,095,679; West Germany 2,909,706 and 2,745,150; East Germany 218,76 and 213,226; and Czechoslovakia 212,856 and 210,447. Known processes for producing liquid epoxy resins from bisphenol-A and excess epichlorohydrin are either continuous or discontinuous processes operating in the presence of an alkali metal hydroxide in quantities of 2 moles, or about 2 moles, for every mole of bisphenol-A.
In a typical discontinuous process, a concentrated aqueous solution of alkali metal hydroxide is fed to a solution of bisphenol-A in epichlorohydrin at atmospheric or slightly lower than atmospheric pressure. The temperature is controlled to continuously distill the water introduced with the alkali metal hydroxide as an azeotropic mixture with the epichlorohydrin. After completion of the addition of the solution of alkali metal hydroxide, all the water is removed, the unreacted epichlorohydrin is recovered by distillation at pressures lower than atmospheric, and the alkali metal chloride, a sub-product of the reaction is separated by filtration of the solids or dissolution in water with subsequent dilution of the brine/organic mixture. Liquid epoxy resins obtained in such a process have a high viscosity, an undesirable color, and because of their relatively high chlorine content, are not suitable for various applications. Liquid epoxy resins made this way can have a residual chlorine content of the order of 0.5-0.8% by weight.
Several methods of producing liquid epoxy resins by a continuous process, by effecting the reaction of the bisphenol-A with the epichlorohydrin in a number of reactors installed in series are well known in the art. In such processes, the bisphenol-A and the epichlorohydrin are continuously fed to a first reactor, while the alkali metal hydroxide in aqueous solution is introduced into each reactor up to a maximum quantity equal, or about equal to 2 moles for every mole of bisphenol-A. The reaction products are discharged continuously from the last reactor and are subjected to decantation to separate the liquid epoxy resin from the water and the alkali metal chloride which is a by-product of the reaction.
In various conventional processes, the reaction is carried out in the presence of oxygenated organic substances of alcoholic or ketonic nature. The presence of extraneous substances in these procedures can cause a decrease in the purity of the resin produced, and the reactive substances such as the alcohols or the ketones can give rise to secondary reactions with formation of various sub-products. The added substances are eventually separated from the liquid epoxy resin, and are purified before recycling them to the reaction. The liquid epoxy resin is separated from the water and the alkali metal halide which is a sub-product of the reaction.
In other prior art processes, chemistries for various reactions are carried out in batch reactors with vapor removal and concurrent reactant addition systems. To conduct such reactions in a continuous flow process would require: 1) relatively fast reaction kinetics (e.g. 1 to 2 minutes) in a pipe reactor, or 2) an infinite series of continuous stirred tank reactors (in practice 10 to 20 reactors in series).
In the past, reaction of a dichlorohydrin aqueous intermediate with an alkali metal salt to epoxidize the dichlorohydrin results in side reactions with the water present which give rise to hydrolysis products which include glycerin monochlorohydrin, glycidol, and glycerin itself. These by-products are all undesirable because they are difficult to remove from the aqueous effluent of the process.
There has long been a need for an effective and efficient continuous reactor and for processes employing such a reactor. There has long been a need for an effective and efficient continuous process for producing liquid epoxy resins. There has long been a need for apparatus for use in such processes. There has long been a need for a continuous process for the production of liquid epoxy resins in which reaction by-products may be removed at various stages in the process. There has long been a need for a continuous process for the production of liquid epoxy resins in which catalysts and reactants can be added in a staged manner to minimize yield losses to undesirable side reactions.